This invention concerns a new analogue-to-digital converter architecture, and an analogue-to-digital conversion system embodying said converter.
Analogue-to-digital converters, hereinafter referred to as ADCs, are widely used in many engineering fields for digital processing of information input as analogue data, particularly in telecommunications and radar for which information throughputs are very high.
The two main performances required for the ADC function are:
high dynamics for the signal to be processed; PA1 optimum processing speed that requires a high information input rate, furthermore knowing that to respect Nyquist's criterion, the clock frequency of the circuits used should be at least twice the highest frequency in the spectrum of the signal to be coded. PA1 L is the length of the optical channels, or the interaction length of the electro-optic line; PA1 k is a constant; PA1 V(t) is the analogue voltage to be coded. PA1 electro-optic means of phase modulation on a predetermined interaction length L, receiving an optical input signal and the electric signal to be converted, and outputting a first and second optical signal having with respect with each other a differential phase that varies linearly as a function of said electric signal to be converted; PA1 an opto-electric device into which the first and second optical signals are input and that outputs at least a first and second electric signal, which are functions of the sine and cosine respectively of said differential phase; PA1 coding means to extract absolute values of the first and second electric signals in digital form coded on [N-3] bits, said coding means having maximum coding dynamics adjusted to code absolute values with reference to a differential phase included in a home sector which will be one of the four possible consecutive sectors partitioning the trigonometric circle into 2II radians; PA1 transcoding means outputting said value V, by coding said differential phase on N bits, starting from absolute values coded on [N-3] bits and the given home sector.
Difficulties encountered in the coding domain are related to the fact that it is not easy to obtain good dynamic and good wideband performances simultaneously.
The state of the art of ADCs includes firstly purely electronic converters (ramp ADC, ADC with successive approximations, flash ADC, etc.), which are either fast, or have good dynamics, and secondly electro-optic effect converters that can be very wideband (1 to 1.2 GHz) but which have a relatively poor dynamics (4 bits). The article entitled "Wide-band electro-optic guided-wave analogue-to-digital converters" IEEE, volume 72, No. 7, July 1984, describes a converter of this second type.
The latter type of converter uses a Mach-Zehnder interferometric modulator that consists of an opto-electric crystal coupled at the output with an optical divider that takes an optical input signal and outputs two components with exactly the same power on two optical channels of the same length, and an optical combiner at the output of these channels combines the signals output from them. The modulator also comprises two electrodes, one connected to the earth and the other receiving the analogue signal, for example a voltage, that is to be converted. An electric field E(t) is created as a result of applying the electric signal V(t). The Pockels effect translates the fact that the refraction index of the material used for the electro-optic crystal varies linearly with this electric field E(t). Before recombination, there is a differential phase .phi.(t) between the two optical signals at the output from the two optical channels, expressed by the following relation: ##EQU1## where .lambda. is the wave length of the optical signal at the input to the interferometer;
ADCs with electro-optic effect described in the past and in the literature use several Mach-Zehnder interferometers in parallel and with an interaction length increasing as a power of 2, the signal V(t) to be converted being applied to each of the interferometers. Recombined signals at the output of each interferometer are then demodulated and compared with a common reference limit, in order to generate a binary value that depends on the result of the comparison. All binary values obtained then form the digitized value of the signal V(t).
Another known equivalent alternative is to use several interferometers with the same interaction length in parallel, the interferometers being powered by voltages proportional to the voltage to be converted, and decreasing as a power of 1/2.
However in both cases, the converters obtained have poor dynamics, since one interferometer is necessary for each resolution bit to be obtained.